In recent years, a nonvolatile memory cell storing data by using substances in which a resistance value changes in accordance with external electrical stimulation is manufactured. A memory including the nonvolatile memory cell as stated above is called as a resistance change memory. The substances as stated above can be largely categorized into two types in accordance with electrical characteristics thereof. One may be called as bipolar materials, and the other may be called as unipolar materials.
As the bipolar materials, SrTiO3 and SrZrO3 in which an impurity such as Cr is doped slightly can be cited. Pr(1-x)CaxMnO3, La(1-x)CaxMnO3, and so on representing a colossal magnetoresistance (CMR) are also the bipolar materials. When a voltage having a threshold value or more is applied to a thin film composed of the bipolar material or a bulk material, a change of a resistance occurs. The resistance is kept stable before and after the change. When the voltage having the other threshold value or more with a reversed polarity is applied after the change of the resistance occurs, the resistance of the bipolar material returns to the same level as an original resistance. As stated above, it is necessary to apply the voltages having the polarities different from one another for the change of the resistance, in the bipolar material.
In Non-Patent Document 1, the change of the resistance of SrZrO3 into which Cr of 0.2% is doped is described. When a negative voltage is applied, an absolute value of a current increases rapidly in a vicinity of −0.5 V. Namely, the resistance of SrZrO3 changes rapidly from high-resistance to low-resistance. A switching phenomenon from high-resistance to low-resistance as stated above and a process thereof are sometimes called as a “set”. Next, when the voltage is applied in a positive direction so as to be swept, the value of the current decreases rapidly in a vicinity of +0.5 V. Namely, the resistance of SrZrO3 returns rapidly from low-resistance to high-resistance. A switching phenomenon from low-resistance to high-resistance as stated above and a process thereof are sometimes called as a “reset”. Besides, the respective resistances are stable within a range of ±0.5 V, and they are kept even though the voltage is not applied. Accordingly, it becomes possible to use the bipolar material to a memory by corresponding the high-resistance state and the low-resistance state to “0” (zero) and “1” respectively. Incidentally, a threshold voltage by which the resistance changes depends on a material, a crystallinity, and so on. Besides, it is also possible to change the resistance by applying a pulse voltage aside from a direct-current voltage.
As the unipolar material, an oxide of a transition metal (TMO: Transition Metal Oxide) such as NiOx and TiOx can be cited. In the unipolar material, the change of the resistance occurs independently from the polarity of the applied voltage, and an absolute value of the voltage by which the change from low-resistance to high-resistance (reset) occurs is smaller than an absolute value of the voltage by which the change from high-resistance to low-resistance occurs (set). Besides, the resistance is kept stable before and after the change as same as the bipolar material. Besides, the change of the resistance is reversible. Accordingly, it is possible to switch a value of the resistance without changing the polarity of the voltage. As stated above, it is necessary to apply the voltages mutually having one polarity for the change of the resistance, in the unipolar material. Characteristics of the unipolar material as stated above are described in Non-Patent Document 2. Incidentally, when the pulse voltage is applied, a behavior as stated above can be seen if a pulse width is fixed.
FIG. 12 is a graphic chart illustrating a current-voltage characteristic of a thin film of TiOx being the unipolar material. When a voltage is applied to a high-resistance state thin film (1), the resistance decreases rapidly at a certain voltage (approximately at 1.5 V) and a current increases rapidly (2). After that, when the voltage is lowered while setting a current limit (a limit value: 20 mA) (3), the current returns to zero while keeping the low-resistance state (4). The resistance of the thin film changes from high-resistance to low-resistance by the processing as stated above. Namely, a “set” process appears. This low-resistance state is kept even though the voltage is not applied. Incidentally, the reason why the current limit is set is because a large current may flow in the thin film to be broken without the current limit.
On the other hand, when the voltage is applied to the low-resistance state thin film (5), the resistance increases rapidly at a certain voltage (approximately at 1.2 V), and the current decreases rapidly (6). After that, the current returns to “0” (zero) while keeping the high resistance state if the voltage is lowered (7). The resistance of the thin film changes from low-resistance to high-resistance by the processing as stated above. Namely, a “reset” process appears. This high-resistance state is kept even though the voltage is not applied.
Accordingly, it becomes also possible to use the unipolar material as a memory by corresponding the high-resistance state and the low-resistance state to “0” (zero) and “1” respectively. Namely, in an example illustrated in FIG. 12, it is possible to identify the resistance of the unipolar material from the current value when the voltage of approximately 0.2 V is applied, and to identify which is stored either “0” (zero) or “1” from the identified resistance.
The changes of the resistances of the bipolar material and the unipolar material as stated above do not appear from just after a formation of the thin film and so on, but appear after a dielectric breakdown is occurred by applying a relatively large voltage to the thin film and so on, or a phenomenon resembling to the dielectric breakdown occurs. The phenomenon and the process thereof occurred by the processing as stated above are sometimes called as a “forming”. It is considered that a conductive region called as a filament is generated by the forming process, and the resistance changes caused by a change of the characteristics in the filament. The changes of the characteristics of a TiOx film in each processing are illustrated in FIG. 13A to FIG. 13C. FIG. 13A is a graphic chart illustrating a change of a current in the forming process, FIG. 13B is a graphic chart illustrating the change of the current in the set process, and FIG. 13C is a graphic chart illustrating the change of the current in the reset process. Incidentally, arrows in the respective graphic charts represent directions of the changes of the currents.
As illustrated in FIG. 13A, the resistance of the TiOx film just after it is formed is high, and the current does not increase rapidly without applying the voltage of approximately 8 V in the forming process. Incidentally, the current limit (the limit value: 10 mA) is also set in the forming process.
The voltage is applied again to the TiOx film of which forming process is completed, then the resistance decreases rapidly when the voltage of approximately 2.5 V is applied as illustrated in FIG. 13B. The low-resistance state is kept even though the applied voltage is set to be zero. Namely, the processing of the set process is performed.
After that, the voltage is applied again to the TiOx film of which set process is completed, then the current increases quickly as illustrated in FIG. 13C, but the resistance increases rapidly when the voltage of approximately 1.2 V is applied. The high-resistance state is kept even if the applied voltage is set to be zero. Namely, the processing of the reset process is performed.
After that, the set process or the reset process appears in accordance with the value of the resistance and the applied voltage. Incidentally, there is a case when not the set process but the reset process appears just after the forming process because the resistance of the TiOx film of which forming process is completed is low depending on a value and so on of the limited current at the time of the forming process. After that, only the set process or the reset process appears in accordance with the value of the resistance and the applied voltage, also in this case. Namely, the forming process appears only first one time without applying a thermal stress and so on.
A resistance change memory is expected as an alternative memory of a flash memory of which scaling limit is approximating. However, it is necessary to apply a high voltage for the forming process of TiOx as stated above. Accordingly, an appropriate selection and so on of materials is necessary. It is verified that the forming process completes with a lower voltage than TiOx when NiOx is used, from a result of an experiment performed by the present inventors and so on. Accordingly, it is conceivable that the usage of NiOx is effective.
However, the present inventors have found out that there is a tendency in which the voltage necessary for the forming process (forming voltage) becomes high as the memory cell becomes small, and that it is difficult to decrease the forming voltage to a degree matches with a driving voltage of the other elements even when NiOx is used.
Patent Document 1: Japanese Laid-open Patent Publication No. 2004-241396
Patent Document 2: Japanese Laid-open Patent Publication No. 2004-363604
Non-Patent Document 1: A. Beck et al., Apply. Phys., Lett. 77, 139 (2001)
Non-Patent Document 2: I. G. Baek et al., Tech. Digest IEDM 2004, p. 587